When smoke keeps the windows shut for weeks, a house designed to breathe becomes a house that bakes. A Chiang Mai architecture practice – Niwat Architects – explains how to keep one cool and its air clean, and what the evidence says about the equipment that helps.
Smoke season is no surprise in Chiang Mai, but this year’s was hard to miss. Lowy Institute reporting found that pollution reached disastrous levels from late March into April, and that Chiang Mai often ranked first on IQAir’s list of the world’s most polluted cities. The build-up began earlier. On 3 and 4 February, Citylife recorded morning PM2.5 readings of 72 micrograms per cubic metre in Mae Ai and 65 at Nakornping Hospital, against a Thai standard it cited of 37.5. By April, the provincial governor had ordered a complete ban on burning across all 25 districts.
The causes are structural. NASA’s Earth Observatory reports that, by one estimate, about 70 percent of the fine particles (PM2.5) in Chiang Mai’s April air come from burning crop waste and other biomass, and that the mountains around the city help trap smoke in the valleys. For a sense of scale, the World Health Organization’s 2021 air quality guidelines recommend a 24-hour PM2.5 level of 15 micrograms per cubic metre and an annual level of 5.
For homeowners, the question is practical. What should the house itself do about it? We put it to Khun Pawat of Niwat Architects, a Chiang Mai practice that lists green architecture and passive cooling among its specialities in the GoldenPages directory.
The trouble with a house built to breathe
Many Chiang Mai homes are designed to open up and let the breeze through. It is a sound idea for most of the year and the wrong one for smoke season. Khun Pawat says homes designed for natural airflow become very uncomfortable to live in once the smoke arrives.
It is not hard to design a house that stays comfortable without relying on air conditioning.
– Khun Pawat, Niwat Architects
The difficulty is the closing up. In smoke season, doors and windows have to be shut tightly to keep dust out, and that also stops the breeze passing through. The US Environmental Protection Agency gives the same basic advice: keep windows and doors closed during smoke, and use fans and air conditioning to stay cool. A sealed house traps whatever heat it already holds, which is why the design of the shell matters as much as any machine inside it.
Six ways to keep a closed house cooler
Khun Pawat lists six measures.
- Plant trees for shade.
- Add insulation under the roof to reduce the heat that builds up there.
- Use fans to move the air around inside the house.
- Choose light, reflective colours that do not absorb heat.
- Install a fine water mist system around the house to add humidity. Ask your designer how much added moisture suits your house.
- Choose doors, windows and glass that block heat and seal tightly when closed.
Read together, the list works on two fronts. Trees, roof insulation, pale colours and better glazing cut the heat that reaches the house in the first place. Fans and mist deal with the air that is already inside. The sixth measure does double duty, since the same tight seals that keep heat out also keep smoke out.
Clean air is the most important part
Of everything on his list, Khun Pawat singles out one element as the most important: a positive pressure system, so that the air inside the house is clean. In plain terms, a positive pressure system uses a fan to keep the air pressure inside slightly higher than outside, so air leaks out through small gaps instead of smoke leaking in.
One caution applies to anything that draws outdoor air into the house. For homes with a fresh air intake, the EPA advises closing it or switching to recirculation during heavy smoke, and relying on a high-efficiency filter such as MERV 13 and a portable air cleaner to clean the indoor air. A system that pulls in outdoor air therefore needs good filtration on that air, so ask your designer how it is handled.
Where the money goes
According to Khun Pawat, replacing the doors, windows and glass, and adding a positive pressure system, makes the biggest difference. There are many options at different price levels. He says indoor air can stay much cleaner than the air outside when a house is sealed tightly and has such a system.
Is it worth the money? Khun Pawat says that depends on personal factors, so he cannot say which choice is not worth making. That is a fair answer. A household with a young child or an elderly relative will weigh the cost differently from a couple who travel during the worst weeks.
A cheaper layer: portable air cleaners
Building work is not the only route, and it is rarely the quickest. A portable air cleaner in the room where the household spends most of its time is the step the EPA itself points to. Three points from public-health guidance are worth knowing before you buy.
- Match the unit to the room. King County Public Health in Washington State suggests a clean air delivery rate (CADR) of 65 for every 100 square feet, roughly 9 square metres, assuming an 8 foot ceiling. A 200 square foot room, about 19 square metres, would need a CADR of 130.
- Be wary of ionisers and similar features. The same guidance advises avoiding units with ionisation, UV lights or oil diffusers unless third-party verified, because some produce ozone.
- Plan for upkeep. Filters need replacing, and the guidance recommends wearing an N95 or KN95 mask and removing dirty filters outdoors.
For a lower-cost option, Penn State Extension describes the Corsi-Rosenthal box, a box fan with four or five MERV 13 furnace filters arranged around it. Citing Rutgers University research, it reports that a four-filter unit achieved a CADR of about 359 cubic feet per minute and that MERV 13 filters cleaned about 71 percent better than MERV 8. It is a supplement for a room, not a substitute for a sealed house, and the extension service says it is no replacement for medical advice during severe smoke events.
A sensible order of work
- Check the doors, windows and glass first. Tight seals and heat-blocking glass come up several times in Khun Pawat’s advice.
- Ask a qualified architect or engineer to price a positive pressure system, and ask how the air it draws in is filtered.
- Add shade, roof insulation and fans to manage the heat inside.
- Put a correctly sized air cleaner in the main living room and bedroom while longer-term work is planned.
- Watch outdoor air on IQAir’s live Chiang Mai page so you know when to keep the house sealed.
Questions to ask before you sign
- How is the air that the system draws in filtered, and how often does the filter need changing?
- How tightly will the new doors and windows seal when closed, and how is that checked?
- What happens to the house’s temperature when the windows are shut and the system is running?
- What is the running cost, including filters and electricity?
If you are choosing professionals for this kind of work, start with the GoldenPages directory, which lists architects, interior designers and air conditioning services.
